The present invention relates to fastening machines and in particular to improved aspects of fastener delivery to and around a fastening machine including a method for the controlled and efficient flow of fasteners from their point of manufacture to their insertion in a workpiece.
The term “fastener” is used herein to include rivets, screws, slugs and other types of fastening devices.
Conventionally rivets are presented to a fastening machine in loose form (e.g. they are delivered to the site in a bag which is severed and unloaded into a hopper of the machine) or mounted in a carrier tape. In the former design the rivets are extracted singly from the hopper and delivered to a rivet setting tool via a pressurised delivery tube in which the rivet is propelled by, for example, pressurised air. At the end of the delivery tube the rivet is typically transferred to an alignment or retaining device for holding the rivet in alignment with a rivet delivery passage of the setting tool. When the rivet is in this position a punch descends along me rivet delivery passage and drives the rivet into the workpiece so mat it is deformed by an upsetting die disposed below the workpiece. In designs which use carrier tape the fasteners are advanced with the tape so that they are brought sequentially into alignment with the punch and die assembly by a feeder before the punch is actuated to drive the fastener out of the tape and into the workpiece as before.
In certain applications where limited space is available the use of a conventional carrier tape and feeder design is precluded by their size.
Modem riveting machines are generally CNC controlled and incorporate robot technology. The machines are operated under the control of a computer program that provides instructions relating to the rivet position and type for each joint to be effected in a particular workpiece. The type of rivet to be used is selected according to many factors including the size of the parts to be connected. The fastener delivery system must thus be able to cope with the supply of rivets of different sizes and types in any particular sequence without increase to the riveting cycle time.
A present requirement in the industry is to meet the demands of large scale continuous production in which setting tools are supplied in a continuous uninterrupted manner both during operation of the setting tool and during robot dwell times when the setting tools are not in operation. In such fastening machines rivets are preferably transferred in bulk from a store or goods inward station to the setting tool on a production line in a “Just-in-Time” manner by automatic means such as, for example, auto-guided vehicles, robots or conveyors.
A problem with presenting loose rivets or other fasteners to conventional fastening machines is that the supply hopper or other storage device is topped up from time to time with fasteners that can be from different production batches, making it impossible to trace with any accuracy the passage of individual rivets or batch of rivets from the source of manufacture through to insertion in the workpiece. The mixing of batches compromises strict quality control measures demanded by modern industry, especially in the event of having to recall a riveted product. Operator error or non-compliance with procedures (e.g. adding rivets from an unidentifiable source to a feeder containing identifiable rivets) can exacerbate this difficulty.
A disadvantage of existing rivet delivery tubes is the tendency for them to wear during use because the plastics material from which they are generally constructed is selected as a compromise between flexibility, visual transparency (so that blockage or jams can be detected by visual inspection) and a low coefficient of friction. This is particularly so if rivets are fed sideways (i.e. at right angles to the longitudinal axis of the rivet) which is necessary if tumbling of the rivet within the tube is to be avoided. Fasteners having different aspect ratios (fastener length to head diameter) are fed in different orientations. For example, fasteners with a low aspect ratio are susceptible to tumbling in the delivery tube, which must therefore be of T-shape, or rectangular cross-section and fasteners with a high aspect ratio are transported axially in tubes of circular cross-section. Wear can manifest itself in the form of internal corrugations that can severely limit the propulsion velocity. In addition, the accumulation of dust and general detritus can cause blockages thereby interrupting the fastening process particularly as it is generally difficult to gain access to the interior of the tube. Such delivery tubes are generally connected to robotic devices and can be twisted or otherwise contorted during robot manipulation, particularly when routed around a bend having a small radius. In such cases the inner profile of the tube can be distorted to an extent that rivets become trapped in a constriction in the tube.
Another problem with sideways delivery of rivets is that they need to be rotated through 90° before they can be inserted into the delivery passage of the nose when the delivery tube approaches the nose from a vertical direction that is parallel to the setting tool axis. This can be done by incorporating bends into the delivery tube or feeder tube of a transfer station however this occupies considerable space since the bend must be gradual enough so to prevent jamming of the rivet and to maintain sufficient rivet momentum. Generally die transfer station has a plunger mat directs a rivet emerging from the delivery tube into the nose of the setting tool. The delivery tube must therefore enter the transfer station ahead of the plunger in which case the tube must bend around the plunger, or the plunger must be constructed so as to reciprocate out of the path of the tube when a rivet arrives.
In certain fastening applications several rivet sizes are required for a workpiece or section of a workpiece if, for example, it comprises overlapping sheets or there is a requirement to attach a bracket to another component, in which case the sandwich thickness of the workpiece varies from two sheets to three sheets or more. When self-piercing riveting technology is employed, one of the factors determining the strength of a riveted joint is the length of the rivet in relationship to the sandwich thickness of the material to be fastened. The mechanical properties of joints riveted with the same size of rivet will vary depending on the sandwich thickness and the material being fastened. In a continuous production environment, conventional self-piercing riveting tools are dedicated to a single rivet size and the problem of riveting combinations of different thicknesses of material is addressed by using several dedicated tools each applying a different rivet size. Obviously this requires careful planning as increased combinations of different joint thicknesses and strengths require additional rivet sizes and therefore increased numbers of tools.
Finally, it is a continual requirement to improve the efficiency and reliability of the transfer of individual rivets from the delivery tube to the rivet delivery passage in the setting tool.
In many known setting tools rivets are transported directly into the nose via a permanently connected delivery tube. This arrangement has several disadvantages. In particular, the connection of the tube to the nose restricts access, is bulky and means that the tube must move up and down with the stroke of the nose during insertion of a rivet into a workpiece. Moreover, the rivet delivery can be a problem in that there is no provision for dealing with a plurality of rivets that may have been accidentally fed into the nose and effective delivery relies purely on the momentum of the rivet as it travels down the delivery tube. It will be understood that the rivet momentum is variable with the air pressure supply (that propels the rivets along the tube), rivet mass and restrictions in the passage of the delivery tube (caused by kinks, bends, dirt and wear etc). In addition, the arrangement cannot prevent debris being carried into the nose along the delivery tube.
In applications where there is restricted access to a workpiece long slender noses are used and the rivet entry passage has to be positioned high up the nose so that long strokes of the punch within the nose are required. This increases the cycle time and adds significantly to the overall length of the setting tool.
Finally, there is generally a slow cycle time associated with such transfer arrangements. Rivets are fed separately to the nose and the cycle time is thus dependent on the length of the delivery tube.
In an alternative known configuration a transfer station is disposed between the nose and die delivery tube. Rivets stop at the transfer station and are transferred by a pusher into the nose. Whilst this arrangement reduces the cycle time in that rivets can be collected at the transfer station, the other disadvantages referred to above are not solved.
U.S. Pat. No. 5,465,868 describes an automatic system for pre-selecting and feeding pre-oriented rivets to a riveting machine. A buffer magazine comprising a bundle of tubes is situated at a location intermediate a rivet setter head and a feed station. Each tube contains a plurality of rivets The buffer magazine is supplied with pre-oriented rivets of different sizes and types and is connected to the rivet setter head by a plurality of delivery tubes that are fed by a selecting device mounted on a frame below the magazine. The selecting device operates under the control of a computer program to select the appropriate rivet from the magazine and release it into the appropriate delivery tube for supply to the rivet setter head. The feed station ensures that the buffer magazine is automatically filled to a level above a minimum.
It is an object of the present invention to obviate or mitigate the aforesaid disadvantages.